Fullerene molecules C60, and C70 shown in FIGS. 15A and 15B were found in a mass spectrometry spectrum of a cluster beam by laser ablation of carbon in 1985 (Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985.318,162.), and 5 years later in 1990, a producing method of the fullerene molecules by arc discharge method using a carbon electrode was established. Since then, attention has been focused on the fullerene molecules as a carbon-based semiconductor material or the like.
Moreover, the first example of synthesis of an compound with a structure in which a plurality of hydroxyl groups are added to at least one carbon atom of a fullerene molecule, that is, polyhydroxylated fullerene (which is commonly called “fullerenol”, hereinafter referred to as fullerenol) was reported in 1992 by Chiang et al. (Chiang, L. Y.; Swirczewski, J. W.; Hsu, C. S.; Chowdhury, S. K.; Cameron, S.; Creegan, K. J. Chem. Soc, Commun. 1992, 1791 and Chiang, L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled, S.; Cameron, S. J. Org. Chem. 1994, 59, 3960). Since then, attention has been focused on fullerenol into which a certain amount or over of the hydroxyl groups is introduced, specifically its water-soluble property, and the fullerenol has been studied mainly in technical fields related to biotechnology.
Further, a compound in which the hydroxyl groups of the above-described fullerenol was replaced with sulfone groups, that is, hydrogensulfate-esterified fullerenol was reported in 1994 by Chiang et al. (Chiang, L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled, S.; Cameron, S. J. Org. Chem. 1994, 59, 3960).
FIG. 16 shows an example of a conventionally known method of synthesizing the fullerenol and the hydrogensulfate-esterified fullerenol.
In the conventionally known synthesizing method (Long Y. Chiang et al. J. Org. Chem. 1994, 59, 3960), fuming sulfuric acid is added to the fullerene molecule, then the fullerene molecule is hydrolyzed so as to obtain fullerenol C60(OH)n. When the fullerenol reacts with sulfuric acid, hydrogensulfate-esterified fullerenol C60(OSO3H)n is produced.
In recent years, for example, as a solid high molecular weight electrolyte type fuel cell for a vehicle's power source, a fuel cell using a proton (hydrogen ion; hereinafter referred to as the same) conducting high molecular weight material such as perfluorosulfonic acid resin (Nafion(R) of Du Pont or the like) is well known.
Moreover, as a relatively novel proton conductor, a polymolybdic acid containing a large amount of hydrated water such as H3Mo12PO40.29H2O, or an oxide containing a large amount of hydrated water such as Sb2O5.5.4H2O or the like is well known.
When the high molecular weight material and the hydrated compounds are placed in a wet state, they exhibit high proton conductivity at about room temperature. In other words, when the perfluorosulfonic acid resin is taken as an example, protons ionized from a sulfonic acid group of the perfluorosulfonic acid resin are bonded (hydrogen-bonded) to water contained in, for example, a high molecular weight matrix of the solid high molecular weight electrolyte in a large amount, to produce protonated water, that is, oxonium ions (H3O+), and protons in the form of oxonium ions can smoothly move in the high molecular weight matrix, so a matrix material of this kind can exert a very high proton conduction effect even at room temperature.
On the other hand, recently, a proton conductor with a conduction mechanism completely different from those of the above proton conductors has been also developed. More specifically, it has been found that a composite metal oxide with a perovskite structure such as SrCeO3 doped with Yb or the like can conduct protons without using water as a transfer medium. It has been considered that in the composite metal oxide, protons are singly channeled between oxygen ions forming a framework of the perovskite structure so as to be conducted.
In this case, conductive protons are not originally present in the composite metal oxide. When the perovskite structure is in contact with water vapor contained in an ambient atmospheric gas, water molecules at high temperature react with an oxygen deficient portion formed in the perovskite structure by doping, and the protons are generated only by the reaction.
However, the above described various proton conductors have the following problems.
Firstly, in order to maintain high proton conductivity, the matrix material such as the perfluorosulfonic acid resin is required to be continuously placed in a wet state during use. Therefore, a humidifier or various accompanying apparatuses are required to be mounted in the entire structure of a system such as fuel cell or the like, so an increase in the size of the system and cost for system configuration is inevitable.
Moreover, the operating temperature of the system is limited to a range in which freezing and boiling of water contained in the matrix do not occur, so there is a problem that it is difficult to have a wider temperature range.
Further, in the case of the composite metal oxide with the perovskite structure, in order that meaningful proton conduction is carried out, the operating temperature is required to be maintained at 500° C. or over.
As described above, the conventional proton conductors have the following problems. The conventional proton conductor has high dependence on atmosphere, and more specifically, moisture must be supplied to the proton conductor, or the proton conductor requires water vapor. Further, the range of the operating temperature is narrow, or the operating temperature is too high.
Therefore, the applicant of the present invention has found that, as described above, fullerenol and hydrogensulfate-esterified fullerenol exhibit proton conductivity, and has proposed novel proton conductors (hereinafter referred to as inventions disclosed in the prior applications) in Japanese Patent Application No. Hei 11-204038 and 2000-058116.
The proton conductors according to the inventions disclosed in the prior applications can be used in a wide temperature range including room temperature, and a lower limit of the temperature range is not specifically high. Further, the proton conductors do not require water as a transfer medium. Therefore, the proton conductors have achieved a reduction in dependence on atmosphere, and an increase in an applicable range.
There are two following factors which control proton conductivity of fullerenol and hydrogensulfate-esterified fullerenol.
One of the factors is a structural aspect. It is considered that a proton transfer phenomenon occurs by quantum channeling effects, so fullerenol and hydrogensulfate-esterified fullerenol preferably have a tightly packed solid structure, because (1) the quantum channeling effects are highly dependent on a distance between each site which transfers protons, (2) when fullerenol and hydrogensulfate-esterified fullerenol have a tightly packed solid structure, a more stable thin film can be formed, thereby a thinner layer with high conductance can be supplied, and (3) a loss of H2 in a proton conducting layer is reduced by diffusion of H2.
The other factor is the number of sites which transfer protons. An important factor which controls conductance is the number of charged carriers which can be used for proton transfer. Therefore, an improvement in proton conductance can be expected by increasing the number of proton transfer sites in the proton conducting layer.
However, in the above-described conventional method of synthesizing fullerenol and hydrogensulfate-esterified fullerenol, the following problems arise. Namely, when the hydroxyl groups are added to a fullerene molecule, the position of the hydroxyl groups introduced into at least one carbon atom of the fullerene molecule cannot be controlled, and the number of the hydroxyl groups introduced into the fullerene molecule cannot be controlled (12 hydroxyl groups per fullerene molecule is a limit).
In view of the foregoing, it is a first object of the invention to provide a method of efficiently producing fullerene into which a hydroxyl group is introduced and which is suitable as a proton conductor, such as fullerenol, or a derivative thereof.
It is a second object of the invention to provide a novel and useful fullerene derivative obtained by the method, a proton conductor, and an electrochemical device using the proton conductor.